U.S. patent number 5,270,880 [Application Number 07/866,687] was granted by the patent office on 1993-12-14 for predictive read-write inhibit decision using fuzzy logic.
This patent grant is currently assigned to International Business Machines Corporation. Invention is credited to Hal H. Ottesen, Arun Sharma, Muthuthamby Sri-Jayantha.
United States Patent |
5,270,880 |
Ottesen , et al. |
December 14, 1993 |
**Please see images for:
( Certificate of Correction ) ** |
Predictive read-write inhibit decision using fuzzy logic
Abstract
A method for predicting and avoiding data errors by predicting
disk drive read-write inhibit requirements in response to at least
two parameters including a head position error and velocity with
respect to the track centerline. A write or read inhibit signal is
generated in response to selected combinations of these parameters
to minimize soft read errors arising from track misregistration and
to avoid hard errors arising from overwriting data recorded in an
adjacent track. The read and write inhibit strategy is described as
a pair of linguistic control rules suitable for implementation in a
Fuzzy Logic Controller.
Inventors: |
Ottesen; Hal H. (Rochester,
MN), Sharma; Arun (New Rochelle, NY), Sri-Jayantha;
Muthuthamby (Ossining, NY) |
Assignee: |
International Business Machines
Corporation (Armonk, NY)
|
Family
ID: |
25348170 |
Appl.
No.: |
07/866,687 |
Filed: |
April 10, 1992 |
Current U.S.
Class: |
360/60;
G9B/19.001; G9B/19.005; 360/77.02; 360/55 |
Current CPC
Class: |
G11B
19/02 (20130101); G11B 19/04 (20130101) |
Current International
Class: |
G11B
19/04 (20060101); G11B 19/02 (20060101); G11B
015/04 () |
Field of
Search: |
;360/60,55,77.02
;369/54 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Fuzzy Logic by Lofti A. Zadeh, Apr. 1988 with IEEE, pp. 83-92.
.
U.S. Fuzzy Logic Comesout of the Closet by R. Colin Johnson,
Electronic Engineering Times 1991 pp. 19 & 76. .
Fuzzy Logic Simplifies Complex Control Problems by Tom Williams,
Mar. 1991 with Computer Design, pp. 90-100..
|
Primary Examiner: Look; Edward K.
Assistant Examiner: Lee; Michael S.
Attorney, Agent or Firm: Baker, Maxham, Jester &
Meador
Claims
We claim:
1. A method for controlling the read-write error rate in a
recording disk file of the type having a read-write head means and
a plurality of data storage tracks, each said track bounded by
inner and outer read offtrack limits (ROLs) separated by a ROL
width and inner and outer write offtrack limits (WOLs) separated by
a WOL width, said limits disposed about a track centerline, wherein
said head means has a head position error with respect to said
centerline and is connected to a servo-actuator means for moving
said head means inward or outward with respect to each said track
over a track-follow velocity range having an upper head velocity
magnitude limit (VML), said method comprising the steps of:
establishing a first fuzzy set membership function defining the
degree of membership of said head position error in a first fuzzy
set;
establishing a second fuzzy set membership function defining the
degree of membership of said head velocity in a second fuzzy
set;
combining said first and second fuzzy set membership functions
according to a first lexical rule to form a read inhibit decision
function;
combining said first and second fuzzy set membership functions
according to a second lexical rule to form a write inhibit decision
function;
generating a read inhibit signal in response to a value of said
read inhibit decision function greater than a first threshold;
generating a write inhibit signal in response to a value of said
write inhibit decision function greater than a second threshold;
and
inhibiting the read and write functions of said head means in
response to the corresponding said read and write inhibit
signals.
2. The method of claim 1 wherein:
said first lexical rule is to inhibit said write function if either
one of said head position error or said head velocity is at least
medium and the other is at least small in the same direction inward
or outward; and
said second lexical rule is to inhibit said read function if said
head velocity is large, or if said head velocity is at least small
and said head position error is large in the same direction inward
or outward, or if both said head position error and said head
velocity are medium in the same direction inward or outward.
3. The method of claim 2 wherein:
said first fuzzy membership function value is around zero for said
head positions within one-sixth of said WOL width of said
centerline, positive small for said head positions from one-sixth
to one-third of said WOL width outward of said centerline, negative
small for said head positions from one-sixth to one-third of said
WOL width inward of said centerline, positive medium for said head
position errors from one-third to one-half of said WOL width
outward of said centerline, negative medium for said head position
errors from one-third to one-half of said WOL width inward of said
centerline, positive large for said head positions from said outer
WOL to said outer ROL, and negative large for said head positions
from said inner WOL to said inner ROL; and
said second fuzzy membership function value is around zero for said
head velocity magnitudes less than one-third of said VML, positive
small for outward head velocities between one-third and two-thirds
of said VML, negative small for inward head velocities between
one-third and two-thirds of VML, positive medium for outward head
velocities between two-thirds and three-thirds of said VML,
negative medium for inward head velocities between two-thirds and
three-thirds of said VML, positive large for outward head
velocities greater than said VML and negative large for inward head
velocities greater than said VML.
4. The method of claim 3 wherein said data storage tracks are for
storing magnetic data and said head means is for reading and
writing magnetic data in said data storage tracks.
5. The method of claim 1 wherein said data storage tracks are for
storing magnetic data and said head means is for reading and
writing magnetic data in said data storage tracks.
6. The method of claim 1 wherein said data storage tracks are for
storing optical data and said head means is for reading and writing
optical data in said data storage tracks.
7. A data recording disk file of the type having at least one
rotatable disk with substantially concentric data storage tracks, a
head means for reading and writing data in said tracks during disk
rotation, a servo-actuator means attached to said head means for
causing said head means to follow a specific data storage track in
response to a driver signal, a signal conditioner and driver means
for providing said driver signal in response to a state controller
output signal, and demodulator means for deriving and generating a
head position error signal, said disk file further comprising:
estimator means for generating a head velocity signal in response
to said state controller output signal and said head position error
signal;
state controller means for generating said state controller output
signal in response to said head velocity signal and said head
position error signal;
inhibit controller means for generating a read inhibit signal and a
write inhibit signal in response to said head velocity signal and
said head position error signal; and
read-write channel means for inhibiting said read and write
functions of said head means in response to said read and write
inhibit signals.
8. The disk file of claim 7 wherein said inhibit controller means
comprises:
fuzzy logic means for converting said head velocity signal and said
head position error signal into said write inhibit signal according
to a first lexical rule; and
fuzzy logic means for converting said head velocity signal and said
head position error signal into said read inhibit signal according
to a second lexical rule.
9. The disk file of claim 8 wherein:
said first lexical rule creates a write inhibit signal output when
either one of said head position error or said head velocity signal
is at least medium and the other is at least small in the same
direction inward or outward; and
said second lexical rule creates a read inhibit signal when said
head velocity is large, when said head velocity signal is at least
small and said head position error signal is large in the same
direction inward and outward, and when both said head position
error signal and said head velocity are medium in the same
direction inward or outward.
10. The disk file of claim 9 wherein said data storage tracks are
for storing magnetic data and said head means is for reading and
writing magnetic data in said data storage tracks.
11. The disk file of claim 7 wherein said data storage tracks are
for storing magnetic data and said head means is for reading and
writing magnetic data in said data storage tracks.
12. The disk file of claim 7 wherein said data storage tracks are
for storing optical data and said head means is for reading and
writing optical data in said data storage tracks.
13. In a recording disk file of the type having at least one
read-write head means, at least one recording disk including a
plurality of data storage tracks, each of said data storage tracks
bounded by inner and outer read offtrack limits (ROLs) separated by
a ROL width and inner and outer write offtrack limits (WOLs)
separated by a WOL width, said limits disposed about a track
centerline, head position error means for measuring a head position
error with respect to a centerline, a servo-actuator means
responsive to said head position error for moving said head means
inward or outward with respect to each track of said plurality of
data storage tracks over a track-follow velocity range having a
upper head velocity magnitude limit (VML), a method for controlling
the disk file's read-write error rate, the method employing:
a rule-based controller means for making read and write inhibit
decisions in response to a velocity of said head means with respect
to a track centerline and to a position of said head means with
respect to said track centerline;
the method including the steps of:
generating a head velocity signal representing a time rate of
change of a position of said head means with respect to a
centerline of a disk storage track;
providing said head velocity signal to said rule-based controller
means;
at said rule-based controller means, generating a write inhibit
signal in response to a head velocity signal indicating a head
velocity oriented away from said centerline; and
preventing writing on said track by said head means in response to
said write inhibit signal.
14. In a recording disk file of the type having at least one
read-write head means, at least one recording disk including a
plurality of data storage tracks, each of said data storage tracks
bounded by inner and outer read offtrack limits (ROLs) separated by
a ROL width and inner and outer write offtrack limits (WOLs)
separated by a WOL width, said limits disposed about a track
centerline, head position error means for measuring a head position
error with respect to a centerline, a servo-actuator means
responsive to said head position error for moving said head means
inward or outward with respect to each track of said plurality of
data storage tracks over a track-follow velocity range having a
upper head velocity magnitude limit (VML), a method for controlling
the disk file's read-write error rate, the method employing:
a rule-based controller means for making read and write inhibit
decisions in response to a velocity of said head means with respect
to a track centerline and to a position of said head means with
respect to said track centerline;
the method including the steps of:
generating a head velocity signal representing a time rate of
change of a position of said head means with respect to a
centerline of a disk storage track;
providing said head velocity signal to said rule-based controller
means;
at said rule-based controller means, generating a read inhibit
signal in response to a head velocity signal indicating a head
velocity oriented away from said centerline; and
preventing reading from said track with said head means in response
to said read inhibit signal.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the positioning control of the read-write
head in a rotating disk data storage device and, more particularly,
to a positioning method and apparatus that reduces hard and soft
error rates during data storage disk operation.
2. Discussion of the Related Art
Rotating disk data storage devices are widely used in the art,
offering rapid data file access for reading and writing. These
rotating data stores include magnetic disk memories using
servo-actuator driven magnetic head assemblies to access rotating
magnetic platters. They also include the newer optical disk memory
employing a laser read-write head assembly to access write-once or
read-mostly rotating optical disks. The present state of the art
for magnetic data storage disk files can be appreciated by
referring to, for instance, Comstock, et al., "Data Storage on
Rigid Disks (Chapter 2)", Magnetic Recording, Vol. 2: Computer Data
Storage, (Mee, et al., Eds.), McGraw Hill Book Company, New York,
1988.
The method for reading and writing information to the concentric
data tracks in a rotating disk file is subject to data errors
arising from head tracking errors that occur during data storage to
the file and during data retrieval from the file. A method known in
the art for reducing both hard (written) and soft (read) errors
during writing and reading from a storage device is to define a
track centerline and establish limited offtrack regions about each
track centerline on the disk. The use of write offtrack limit (WOL)
and read offtrack limit (ROL) is known in the art for magnetic and
optical electromechanical storage devices. In principle, the
offtrack method provides a threshold measure for inhibiting the
read or write function of the head assembly. That is, the write
function is inhibited (disabled) when the head position exits the
inner or outer WOL. By preventing the writing of information by a
recording head during an overlap into an adjacent track arising
from a large position error, this limit avoids "hard" or
nonrecoverable data errors. The WOLs define a "write limit width"
for the track. Head position errors greater than half of this write
limit width will force a positive write inhibit decision.
Similarly, when the head position exits the read limit region
bounded by two predetermined ROLs, the head read function is
inhibited to prevent "soft" errors during the read operation.
The problem in the art is that these offtrack limits are static
limits that cannot accommodate changes in head-disk dynamics during
read and write operations. For instance, the relative velocity
between the data storage track and the head assembly is not
considered in the read-write inhibit decision. The fixed offtrack
limits can lead to reduced data transfer efficiency because of a
tendency for the head assembly to bounce back and forth across the
limits during track-following operation under some circumstances
with the methods known in the art. Repeated read operations needed
to correct soft errors can significantly slow data transfer
rates.
Practitioners in the art have developed many useful techniques for
improving the steady-state track-following operation in a rotating
disk storage device. For instance, in U.S. Pat. No. 4,554,652,
Maeda et al. disclose an optical information processor that
ascertains radial movement of the optical read-write spot with
respect to the track centerline using conventional split threshold
methods. Maeda et al provide means for inhibiting the optical
writing operation in response to optical head position errors
beyond a certain Write Offtrack Limit.
In U.S. Pat. No. 4,730,290 Takasago et al. disclose a tracking
error detecting circuit for detecting a laser beam position error
from the centerline of a data storage track in an optical
read-write data storage device. Takasago et al inhibit optical
writing after a position error threshold is first exceeded for a
first predetermined time and the offtrack condition next continues
for a second predetermined time that is longer than the first
predetermined time.
In U.S. Pat. No. 4,839,751, Revels discloses a fine positioning
scheme for a track-following servo-actuator in a disk file. Revels
teaches a two-threshold tracking method where head position errors
that violate either a near-boundary or a far-boundary for one of
several prescribed time intervals are interpreted to indicate
whether the head assembly is "on-center" or not and to logically
exclude noise and other error conditions affecting the "on-center"
indication. Revels provides for variation in threshold timing
intervals to accommodate noise variations but does not consider
inhibit decision modification responsive to changes in head
position error or velocity.
In U.S. Pat. No. 4,954,907, Takita discloses an improved head
positioning control system for track-following in a data recording
disk file. Takita teaches the use of a head velocity detector for
determining head velocity with respect to the data storage track
centerline. He then computes a new servo-actuator input current
signal based on changes in incremental head position and velocity
and the previous servo-actuator input current. Takita's teachings
rest on the assumption that any change in head acceleration that is
not reflected in driver current changes must be entirely a result
of shock or vibration. His system operates to damp the effects of
such unwanted accelerations. Takita does not consider any novel
read-write inhibit decisions for reducing hard or soft errors
during his novel track-following procedure.
Because of the prevailing practice of using hard threshold
read-write inhibit techniques in rotating data storage files, there
is a strongly felt need for improved track-following methods that
will reduce soft error rates and improve data transfer efficiency
during the read and write operations. The related unresolved
problems and deficiencies are solved by this invention in the
manner described below.
SUMMARY OF THE INVENTION
This invention improves the soft error rate through the application
of fuzzy logic technology to the read-write inhibit control
strategy. The essential elements of the inhibit controller of this
invention are the lexical or linguistic control rules relating the
two concepts of fuzzy implication and the compositional rule of
inference. That is, the inhibit controller provides a fuzzy logic
process that converts the lexical control rules arising from expert
knowledge into an automatic control strategy. A set of lexical or
linguistic control rules is converted to an automatic control
strategy through the use of IF . . . THEN type logic
statements.
This invention is based in part on the recognition that the
improved read-write inhibit strategy of this invention can be
described as a set of lexical control rules. These rules are then
converted into an automatic control or decision strategy and
thereby implemented in fuzzy logic as an inhibit controller. The
resulting fuzzy controller process, which may include multiple
inputs and outputs, appears as a multi-dimensional look-up
table.
The improved read-write inhibit strategy of this invention is based
on the recognition that head velocity estimates from head position
error information monitored over time are unexpectedly useful for
improving data transfer efficiency. The read-write inhibit decision
of this invention is made in response to both head position error
information and head velocity information. The method of this
invention for developing the inhibit decision from the head
position error and head velocity information preferably uses a
fuzzy logic approach to convert lexical rules to an automatic
control table, but may also be implemented with conventional
bilevel logical techniques.
It is an object of this invention to reduce the soft error rate
during the read operation by improving the read inhibit decision
procedure, thereby improving data transfer efficiency. It is
another object of this invention to avoid all hard errors during
the write operation without reducing data transfer efficiency by
improving the write inhibit decision procedure. It is a feature of
this invention that both soft and hard error rates are improved
through the use of a simple inhibit control technique incorporating
both head position error and head velocity information as
inputs.
The foregoing, together with other features and advantages of this
invention, will be become more apparent when referring to the
following specification, claims and accompanying drawing.
BRIEF DESCRIPTION OF THE DRAWING
For a more complete understanding of this invention, reference is
now made to the following detailed description of the embodiments
illustrated in the accompanying drawing, wherein:
FIG. 1 shows the inner and outer offtrack limits of a data storage
track from the prior art;
FIG. 2 provides two illustrative fuzzy set membership functions for
head position error and head velocity;
FIG. 3 illustrates the head position fuzzy set membership function
preferred for this invention;
FIG. 4 illustrates the head velocity fuzzy set membership function
preferred for this invention;
FIG. 5 shows the read-write inhibit decision strategy resulting
from lexical rules which are based only on head position;
FIG. 6 shows the read-write inhibit decision strategy resulting
from the lexical rules of this invention for the fuzzy set
membership functions of FIGS. 3 and 4;
FIG. 7 provides an illustrative embodiment of the apparatus of this
invention; and
FIG. 8 shows the Soft Error Rate (SER) improvement arising from the
read-write inhibit strategy of this invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Principles of Fuzzy Logic
This invention involves the application of fuzzy logic techniques
to the read-write inhibit decision process. While fuzzy logic
methods are known, they have not been applied to the rotating disk
storage file art until now. Fuzzy logic is an extension of
traditional bilevel logic. While bilevel logic requires a statement
or condition to be either completely true or completely false,
fuzzy logic permits partial truth and partial falseness. Fuzzy
logic is derived from the more general theory of fuzzy sets. The
degree of membership of a variable value in a fuzzy set can range
from zero to unity with zero indicating null membership in the set
and unity indicating full membership in the set. This function is
exemplified in FIG. 2 for head position error and head velocity
membership in an "inhibit" set of all errors or velocities for
which an inhibit decision must be made.
The mathematical precision of fuzzy sets derives from the precise
mapping of input values to degrees of membership. The linguistic
power of fuzzy logic lies in its ability to define and manipulate
sets that contain varying degrees of membership without the burden
of considering all possible combinations of values. The only thing
vague about fuzzy logic is the linguistic expression of a problem
and its solution, not the numeric representation. A vague or
general linguistic expression of a problem permits the user to
develop and refine a numerical representation without requiring
consideration or understanding of a detailed numeric model. Fuzzy
logic can be applied to complex logical and combinatorial problems
for which numerical models are impossible because of the enormous
number of possible combinations.
A fuzzy logic system differs from a traditional expert system in
that it has far fewer rules to evaluate. A fuzzy logic system
should act to maintain the illusion that all rules in its rule base
are simultaneouly evaluated. This means that complex real-time
systems having large numbers of random interrupts are not suitable
for fuzzy logic solutions. Only systems having smoothly-varying
inputs or outputs are generally suitable for fuzzy logic
solutions.
Fuzzy logic membership functions for several variables can be
combined to form new membership functions using operators similar
to those known for bilevel logic operations. The standard fuzzy
operations are AND, OR and NOT, which correspond respectively to
the INTERSECTION, UNION and COMPLEMENT operations for fuzzy sets.
These three fuzzy operations provide new membership functions
valued according to the minimum membership degree for AND, the
maximum membership degree for OR, and one minus the degree of
membership for the NOT function. That is, the membership degree of
A and B in a fuzzy set is the lesser of the two individual degree
values for the same set, and so on. Reference is made to Zadeh,
"Fuzzy Logic", IEEE Computer, April 1988, pp. 83-93, for a general
discussion of the fuzzy logic art and for additional
references.
A fuzzy logic system is normally implemented using these three
fuzzy logic operators to describe one or more lexical rules
established by the designer. An example of a lexical rule is "IF
displacement is positive small AND the rate of change of
displacement is zero, THEN the driving force is negative small".
(from Maski Togai, "An Example of Fuzzy Logic Control", Computer
Design, March 1, 1991, p. 93).
In U.S. Pat. No. 4,930,084, Osaka, et al. disclose a vehicle
control system based on the implementation of a fuzzy logic auto
cruise control apparatus. Osaka et al provide a useful background
discussion of the application of fuzzy logic techniques to
electronic control system designs but do not consider rotating disk
file art.
The Heat-Tracking Problem
FIG. 1 illustrates the geometry of the concentric data storage
track from the prior art. The track is nominally located at track
centerline 10 and has a finite width bounded by offtrack limits for
the read and write operations. The Read Offtrack Limits (ROLs) are
illustrated as the inner ROL 12 located toward disk center (not
shown) and the outer ROL 14 located toward disk edge (not shown).
ROLs 12 and 14 define a read limit width about track centerline 10.
Similarly, the inner Write Offtrack Limit (WOL) 16 and outer WOL 18
define a write limit width about track centerline 10. The write
limit width is usually narrower than the read limit width because
write operations outside the offtrack limits can result in
unrecoverable or "hard" errors while read operations outside such
limits are "soft" and may be recovered by rereading the data during
the next revolution of the disk. These soft errors merely slow data
transfer rates without causing loss of data.
As can be appreciated with reference to the Comstock et al
reference cited above, the read-write head (not shown) is forced to
follow track centerline 10 as best as possible during a revolution
of the disk. FIG. 1 shows three possible motion vectors at three
different head locations to illustrate possible head-track position
and velocity relationships.
For instance, consider the three head-track velocity vectors at
head position 20. The vector 22 represents negative or inward
motion of the head with respect to track centerline 10. The vector
24 represents positive or outward motion of the head with respect
to the track centerline 10 and the vector 26 represents no relative
motion between head and track centerline 10. Similar vectors are
illustrated for the two other head positions 28 and 30.
At head position 20, the head is well within both ROLs 12 and 14
and WOLs 16 and 18. If the head moves according to the vector 22,
then it is moving closer to negative WOL 16 but no inhibit action
is immediately required unless it is moving rapidly. Motion at
vector 24 improves the head position error at the next sample or
sector and motion at vector 26 retains the status quo. Thus, at
head position 20, no inhibit decision is required except for high
velocities at vector 22.
Head position 28 is close to inner ROL 12 and outside of inner WOL
16. Thus, a write inhibit decision should have been made before the
head position error exceeded inner WOL 16 rather than waiting until
head position 28 is detected. Also, if vector 32 is high velocity,
a read inhibit decision is predictable. In the prior art, no
provision is made for either such type of predictive inhibit
decision. Similarly, the vector 34 at head position 28 implies no
prediction of a read inhibit decision but must be treated
cautiously. Finally, outward vector 36 actually improves the
marginal position situation and requires no inhibit decision.
The three motion vector examples at head position 30 lead to
similar conclusions by this reasoning. Note that the outward vector
improves the situation at head position 28 whereas the inward
vector improves the situation at head position 30, requiring
additional sophistication in any logical decision strategy using
head velocity as an input.
These observations made in connection with FIG. 1 demonstrate that
the absolute head position error value is not as useful to the
inhibit decision process as a combination of head position error
and relative head velocity (the time derivative of position error).
This second combination improves the inhibit decision strategy by
permitting the prediction of position error threshold crossings
before generation of soft (or hard) data errors.
The Invention
The method of this invention expresses the read and write
strategies as one or more linguistic control rules. A properly
defined strategy is then converted into an automated control or
decision scheme and implemented in a Fuzzy Logic Controller (FLC).
Such a FLC procedure, which may have multiple inputs and outputs,
may be expressed as a multi-dimensional look-up table.
The first step in describing the necessary lexical rules for this
invention is to establish a fuzzy set membership function for the
head position error and head velocity discussed above. FIG. 3
provides an illustrative fuzzy set membership function for head
position error and FIG. 4 provides an illustrative fuzzy set
membership function for head velocity. These two functions are
preferred for this embodiment but may easily be modified in form
with similar useful effect. FIG. 2 provides another example of two
such functions relating position and velocity to an "inhibit"
decision.
In FIG. 3, the horizontal axis represents the head position
relative to track centerline and extends inward to the left and
outward to the right. The RL axis labels represent the read
offtrack limit and the WL axis labels represent the write offtrack
limit. WL is divided into thirds on both sides of the track
centerline as shown. For example, the WL/3 distance 38 represents
one-sixth of the width between .+-.WL. The distance between RL and
WL is not necessarily related to the width of the region between
.+-.WL.
The vertical axis in FIG. 3 is labelled with a series of two-letter
symbols representing the fuzzy regions assigned to the
corresponding head position error values shown. For instance, the
"nearly zero" region is labelled ZE and represents the inner third
of the width between .+-.WL. The PS and NS labels represent
Positive and Negative "small error" regions beyond the first third
and inside the last third of the width between .+-.WL. The PS
region is located outward of the track centerline and the NS region
is located inward of the track centerline.
Similarly, PM and NM represent a "medium error" region for head
position errors located in the outer one third of the WOL region.
Finally, "large errors" are labelled PL and NL, representing head
position errors beyond the WOLs that are still within the ROLs.
Membership may also be defined for head position errors beyond the
ROLs for particular design considerations.
Referring to FIG. 4, similar fuzzy set definitions are provided for
the time derivative of head position error, head velocity. Head
velocity is related to a track-following velocity limit, VL. VL is
divided into thirds and each region is labelled using the same
nomenclature as was discussed above in connection with FIG. 3. That
is, "almost zero" is labelled ZE and represents the inner third of
the VL region, "small velocities" are labelled NS and PS
representing the middle third of the VL region, and "medium
velocities" are labelled NM and PM representing the outer third of
the VL region. Again, the positive sign is assigned to outward
velocity and the negative sign is assigned to inward velocity with
respect to the track centerline.
If head velocity falls in the range of PL or NL ("large
velocities"), the cause is likely to be an external mechanical
shock or vibration because the track-following servo control
mechanism does not normally move the head more rapidly than VL.
Accordingly, there is some uncertainty about operating conditions
when head velocity is "large" and both read and write inhibit
decisions could be made under such circumstances. No provision for
this exists in the prior art.
FIG. 5 illustrates a two-dimensional decision table based on the
set membership functions of FIGS. 3 and 4 using fuzzy logic lexical
rules which use only head positions to control the read-write
inhibit decision. The horizontal axis includes two positions at the
extremes labelled "I" to represent head position errors beyond the
ROLs. Such head positions result in automatic inhibition of both
write and read operations, as illustrated in FIG. 5. Similarly, the
PL and NL columns represent head position errors outside of the
WOLs for which the write inhibit decision is always made in this
table. Thus, the PL and NL columns are uniformly filled with "W"
symbols representing write inhibition.
FIG. 6 shows the two-dimensional table from FIG. 5 with the inhibit
decisions resulting from the lexical rules preferred in this
invention. Note that the inhibit decisions in the columns labelled
"I" are identical to those from the prior art, as they should be.
Note that the use of head velocity in the inhibit decision process
permits both the read and write functions to be inhibited for all
"large velocities" in the rows labelled PL and NL. This feature is
a result of the method of this invention incorporating the time
derivative of head position error together with the head position
error in the inhibit decision process. Finally, note that the
inhibit decisions in the other columns vary according to a complex
pattern that results from the lexical rules underlying the inhibit
decision strategy.
Recalling the above discussion in connection with FIG. 1, note that
"large head position errors" (columns PL and NL) require no inhibit
decision when the head velocity is oriented toward the track
centerline. That is, no inhibit decision is predicted or necessary
when head position error and head velocity are of opposite sign.
This refinement is not available in the prior art because head
velocity is not considered.
Inhibit decisions for read and write are more quickly made when
head velocity and head position error are of the same sign, thereby
predicting an increasing problem in the immediate future. Thus, for
combinations of large and small or medium and medium position error
and velocity, both read and write inhibit decisions are made if
velocity and position are in the same direction. This represents
another lexical rule used to define the necessary inhibit
controller logic.
Finally, for combinations of medium and small position error and
velocity in the same direction, no read inhibit decision is made
but the write inhibit decision is made because of a clear
prediction of eventual risk of overwriting outside of the WOLs.
The table in FIG. 6, the set membership functions in FIGS. 4 and 5,
and the above related discussion are intended as illustrative
examples of the effects of using the fuzzy logic membership
functions and lexical rules preferred for this invention. The
division of position error and velocity regions into thirds is
convenient but arbitrary and these methods are just as suitable for
quarter or decimal divisions.
Soft Error Rate (SER) performance is illustrated in FIG. 8 where
the solid line (SER.sub.1) represents the SER of the prior art and
the dotted line (SER.sub.2) represents the improved error rate of
this invention. FIG. 8 shows the SER as a function of head position
error. SER.sub.1 can be either maintained over a wider ROL region
(3.sigma..sub.2) or can be improved to SER.sub.2 over the ROL
region (3.sigma..sub.1).
FIG. 7 presents a conceptual implementation of the predictive
read-write inhibit decision apparatus of this invention. In FIG. 7,
a conventional head positioning servo-actuator 40 is shown
connected to a head assembly 42 by means of the suspension 44. Head
42 is disposed in close contact with a disk 46 and is positioned by
action of servo-actuator 40 in response to the output of a driver
48.
Driver 48 provides a current or voltage output that is proportional
to the output from a digital-to-analog (D/A) converter 50. The
input to D/A converter 50 is a state control output signal 52.
Track position information on disk 46 is sensed by head 42 and
transferred through the arm electronics 54 to a demodulator 56. The
head position error signal 58 is generated by demodulator 56 and
presented to a head velocity estimator 60 and to the state
controller 62, which complete a servo feedback loop. Both estimator
60 and controller 62 may be implemented in a single
microprocessor.
Estimator 60 generates an estimated head velocity signal 64 from
the time rate of change of head position error signal 58. Head
velocity signal 64 is routed to state controller 62 for use in
generating state control output signal 52.
A Fuzzy Logic Controller (FLC) 66 is coupled to this servo loop in
the manner shown. FLC 66 functions as an inhibit controller, which
could also be based on bipolar logic known in the art. The fuzzy
logic implementation is preferred.
FLC 66 produces the two read and write inhibit decision signals 68
and 70 in response to head velocity signal 64 and head position
error signal 58 inputs. FLC 66 is controlled from the R-W channel
72 by data line 74. The character of read inhibit signal 68 and
write inhibit signal 70 can be determined from inspection of FIG.
6.
This invention represents the first known application of fuzzy
logic control methods in a Direct Access Storage Device (DASD). The
fuzzy logic control scheme is limited herein to the read-write
inhibit decision logic. Other DASD control logic may also be
amendable to fuzzy logic techniques.
The method of this invention improves SER because of improved
offtrack detection and prediction capacity. It also reduces
potential hard error conditions arising from inter-track squeezing.
The improved SER results from the capability to predict offtrack
head positioning before it occurs. The method of this invention
also prevents read or write problems during extreme external shock
and vibration and may be used to generate an advisory signal to the
host before loss of data occurs.
The functions of FLC 66 can be incorporated with those of state
controller 62 into a single microprocessor with the necessary
memory for the FLC functions.
Obviously, other embodiments and modifications of this inventions
will occur readily to those of ordinary skill in the art in view of
these teachings. Therefore, this invention is to be limited only by
the following claims, which include all such obvious embodiments
and modifications when viewed in conjunction with the above
specification and accompanying drawing.
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